1. Field of the Invention
The invention relates generally to the utilization of lower alkanes and the synthesis of hydrocarbons. More specifically, the invention relates to conversion of a low molecular weight alkane such as methane to carbon oxides, hydrogen, and water.
2. Description of the Prior Art
The conversion of low molecular weight alkanes (lower alkanes) to synthetic fuels or chemicals has received increasing consideration as such low molecular weight alkanes are generally available from readily secured and reliable sources. Attention has focused on natural gas as a source of low molecular weight alkanes. Low molecular weight alkanes are present in many coal deposits and may be formed during mining operations, in petroleum production and refining processes, and in the gasification or liquefaction of coal, tar sands, oil shale, and biomass.
Accessibility is a major obstacle to effective and extensive use of remotely situated natural gas. Consequently, methods for converting low molecular weight alkanes to chemical feedstocks and to more easily transportable liquid fuels are desired. A number of such methods have been reported which can be conveniently categorized as direct oxidation routes or as indirect syngas routes. The direct oxidation routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, the indirect syngas routes involve the production of synthesis gas (syngas) as an intermediate product.
The direct oxidation routes include oxidative coupling, electrophilic oxidation, oxychlorination, and direct partial oxidation. For example, an article by G. E. Keller and M. M. Bhasin published in the Journal of Catalysis, 73, 1982, 9-19, states that methane can be converted to C.sub.2 hydrocarbons in the presence of reducible metal oxides. The Keller article outlines a reaction method involving the steps of: (1) charging the catalyst with oxygen by passing are oxygen-containing gas over the catalyst; (2) replacing the oxygen in the gas chamber of the catalytic reactor with an inert gas; (3) feeding methane over the catalyst, which partially produces the desired reaction; and (4) supplanting the residual methane and resulting product in the reactor with an inert gas before the sequence of steps is repeated.
The efforts of a number of other researchers in the area of oxidative coupling have been reported. For example:, Jones et al., U.S. Pat. Nos. 4,443,664-9 describe processes for synthesizing hydrocarbons containing as many as seven carbon atoms from methane. In the processes, methane is contacted with a reducible oxide of antimony, germanium, bismuth, lead, indium, or manganese. The aforesaid patents describe ranges of reaction temperature from a lower limit of 500.degree. C. to an upper limit of 1000.degree. C. After reduction, the reducible oxide is reportedly regenerated by oxidation with molecular oxygen. The oxidation may be conducted in a separate zone or by alternating the flow of methane-containing gas with the flow of an oxygen-containing gas.
Additionally, U.S. Pat. Nos. 4,665,260 and 4,560,821 issued to Jones et al. describe an oxidative coupling process in which moving beds of particles containing a reducible oxide of a metal are recirculated between a methane contact zone and an oxygen contact zone. However, efforts at oxidative coupling have bogged down in recent years because of disappointing yields.
Broadly stated, the indirect syngas routes operate by dehydrogenating and oxidizing methane to produce the mixture of hydrogen and carbon oxides known as syngas. Although the ratios of hydrogen and the carbon oxides in syngas may vary widely, all indirect syngas routes require a source of oxygen. For example, U.S. Pat. No. 5,112,527 issued to Kobylinski describes a process for converting natural gas to synthesis gas which utilizes ambient air as a source of oxygen. In the described process, a homogeneous mixture of lower alkanes and air is subjected to partial oxidation and steam reforming reactions in the presence of a first catalyst and water. The product stream from the first catalyst reportedly reacts with water in the presence of a second catalyst having steam reforming activity to produce carbon monoxide and hydrogen. The Kobylinski patent states that the use of air as an oxygen source may result in up to about 45 volume percent nitrogen as an inert component in the gaseous, syngas-containing product.
European Patent Application No. 0399833A1, describes a reactor equipped with separation membranes to exclude nitrogen gas when the reactor is charged with atmospheric air. The reactor reportedly comprises first and second zones separated by a solid multi-component membrane. The '833 application states that such reactors can be used to conduct the partial oxidation of methane to produce unsaturated compounds or synthesis gas.
Researchers have long been intrigued by the prospect of conducting oxidation and reduction reactions within a molten metal bed. An apparatus relating to a process for pyrometallurgically refining copper by passing a reducing hydrocarbon gas through an oxygen-containing molten copper bed is described in U.S. Pat. No. 3,650,519 issued to Vogt et al. The hydrocarbon gas can reportedly be methane, ethane, propane, butane, pentane, or natural gas. The '519 Patent states that injection of methane into a melt of copper containing 0.55 weight percent oxygen produced a gas mixture which, after reacting with the copper melt, was analyzed as having 24 percent CO.sub.2, 13 percent CO, 14 percent H.sub.2, and 3.3 percent CH.sub.4. The '519 Patent does not record any recovery of the gas so produced.
U.S. Pat. No. 4,062,657 issued to Knuppel et al. is directed to a process and an apparatus for gasifying sulphur-bearing coal in a molten iron bath. Reportedly, hot liquid slag is transferred from the iron bath to a second vessel in which the slag is desulfurized by contact with an oxygen containing gas, and then returned to the iron bath for reuse. The '657 Patent notes that under favorable processing conditions a molten iron bath can gasify finely divided coal to produce a combustible gas having an approximate composition of about 70 to 80 percent carbon monoxide and about 15 to about 25 percent hydrogen.
An article by L. Meszaros and G. Schobel in British Chemical Engineering, January 1971, Volume 16, No. 1 describes a molten-bed reactor having a molten lead bath which facilitates the simultaneous oxidation and decarboxylation of furfurol to produce furan. Furfurol and air were reportedly bubbled through molten lead in stoichiometric ratio from a common furfurol-air inlet system and, alternatively, from a separate furfurol inlet and air inlet system. The article states that the method is useful for the partial oxidation of hydrocarbons, alcohols, aldehydes, and for the decomposition of natural gas and gasoline.
U.S. Pat. No. 4,406,666, issued to Paschen et al., is directed to a device for the gasification of carbon-containing material in a molten metal bath process to obtain the continuous production of a gas composed of carbon monoxide and hydrogen. The '666 Patent states that gaseous carbon materials as well as gases containing oxygen can be introduced into the reactor below the surface of the molten metal bath. The molten metal reportedly consists of molten iron, silicon, chromium, copper, or lead.
A method for converting carbon-containing feed, such as municipal garbage or a hydrocarbon gas, to carbon dioxide is described in U.S. Pat. No. 5,177,304 issued to Nagel. The carbon-containing feed and oxygen are introduced to a molten metal bath having immiscible first and second molten metal phases. The '304 Patent states that the feed is converted to atomic carbon in the bath, with the first metal phase oxidizing atomic carbon to carbon monoxide and the second metal phase oxidizing carbon monoxide to carbon dioxide. Heat released by exothermic reactions within the molten bath can reportedly be transferred out of the molten system to power generating means, such as a steam turbine.
U.S. Pat. No. 4,126,668 issued to Erickson presents a process for producing a hydrogen rich gas such as hydrogen, ammonia synthesis gas, or methanol synthesis gas. In the process, steam, carbon dioxide, or a combination of the two is reportedly reacted with a molten metal to produce a molten metal oxide and a gaseous mixture of hydrogen and steam. The '668 Patent states that the steam portion of the gaseous mixture can be condensed and separated to produce a relatively pure hydrogen stream. The molten metal oxide is said to be regenerated for further use by contact with a reducing gas stream containing a reformed hydrocarbon gas, such as reformed methane. When methanol is a desired product, appropriate amounts of carbon dioxide and steam are reportedly reacted with the molten metal, whereby CO.sub.2 is reduced to CO and H.sub.2 O is reduced to H.sub.2 to produce a methanol synthesis gas. Alternatively, the '668 Patent states that the relatively high purity hydrogen stream can be subsequently reacted with CO.sub.2 in a reverse water shift reaction to produce a methanol synthesis gas.
Although the efforts of prior researchers have produced many notable advances in the manufacture of synthesis gas, a need still exists for a new and better process for producing synthesis gas from lower alkanes. Desirably, the process utilizes air as a source of oxygen but also minimizes nitrogen dilution of the synthesis gas product while avoiding entirely the dangers of handling high-purity oxygen. In the interest of safety, the desired process keeps the lower alkanes separated from air at all times. Additionally, the process should be continuous in operation and capable of producing relatively high conversions of methane while maintaining good selectivities for carbon oxides and hydrogen. The process should include opportunities for integrating heat transfer between exothermic and endothermic portions of the process.